Text.XML.Stream.Parse

This module provides both a native Haskell solution for parsing XML documents into a stream of events, and a set of parser combinators for dealing with a stream of events.

As a simple example, if you have the following XML file:

<?xml version="1.0" encoding="utf-8"?>
<people>
    <person age="25">Michael</person>
    <person age="2">Eliezer</person>
</people>

Then this code:

{-# LANGUAGE OverloadedStrings #-}
import Control.Monad.Trans.Resource
import Data.Conduit (($$))
import Data.Text (Text, unpack)
import Text.XML.Stream.Parse

data Person = Person Int Text
    deriving Show

parsePerson = tagName "person" (requireAttr "age") $ \age -> do
    name <- content
    return $ Person (read $ unpack age) name

parsePeople = tagNoAttr "people" $ many parsePerson

main = do
    people <- runResourceT $
            parseFile def "people.xml" $$ force "people required" parsePeople
    print people

will produce:

[Person {age = 25, name = "Michael"},Person {age = 2, name = "Eliezer"}]

Previous versions of this module contained a number of more sophisticated functions written by Aristid Breitkreuz and Dmitry Olshansky. To keep this package simpler, those functions are being moved to a separate package. This note will be updated with the name of the package(s) when available.

Parsing XML files

parseBytes :: MonadThrow m => ParseSettings -> Conduit ByteString m Event

Parses a byte stream into Events. This function is implemented fully in Haskell using attoparsec-text for parsing. The produced error messages do not give line/column information, so you may prefer to stick with the parser provided by libxml-enumerator. However, this has the advantage of not relying on any C libraries.

This relies on detectUtf to determine character encoding, and parseText to do the actual parsing.

parseBytesPos :: MonadThrow m => ParseSettings -> Conduit ByteString m EventPos

parseText :: MonadThrow m => ParseSettings -> Conduit Text m EventPos

Parses a character stream into Events. This function is implemented fully in Haskell using attoparsec-text for parsing. The produced error messages do not give line/column information, so you may prefer to stick with the parser provided by libxml-enumerator. However, this has the advantage of not relying on any C libraries.

detectUtf :: MonadThrow m => Conduit ByteString m Text

Automatically determine which UTF variant is being used. This function first checks for BOMs, removing them as necessary, and then check for the equivalent of <?xml for each of UTF-8, UTF-16LEBE, and UTF-32LEBE. It defaults to assuming UTF-8.

parseFile :: MonadResource m => ParseSettings -> FilePath -> Producer m Event

A helper function which reads a file from disk using enumFile, detects character encoding using detectUtf, parses the XML using parseBytes, and then hands off control to your supplied parser.

parseLBS :: MonadThrow m => ParseSettings -> ByteString -> Producer m Event

Parse an event stream from a lazy ByteString.

Parser settings

data ParseSettings

Instances

Default ParseSettings

def :: Default a => a

The default value for this type.

type DecodeEntities = Text -> Content

psDecodeEntities :: ParseSettings -> DecodeEntities

psRetainNamespaces :: ParseSettings -> Bool

Whether the original xmlns attributes should be retained in the parsed values. For more information on motivation, see:

https://github.com/snoyberg/xml/issues/38

Default: False

Since 1.2.1

Entity decoding

decodeXmlEntities :: DecodeEntities

Default implementation of DecodeEntities: handles numeric entities and the five standard character entities (lt, gt, amp, quot, apos).

decodeHtmlEntities :: DecodeEntities

HTML4-compliant entity decoder. Handles numerics, the five standard character entities, and the additional 248 entities defined by HTML 4 and XHTML 1.

Note that HTML 5 introduces a drastically larger number of entities, and this code does not recognize most of them.

Event parsing

tag

Arguments

:: MonadThrow m

=> (Name -> Maybe a)

Check if this is a correct tag name and return a value that can be used to get an AttrParser. If this returns Nothing, the function will also return Nothing

-> (a -> AttrParser b)

Given the value returned by the name checker, this function will be used to get an AttrParser appropriate for the specific tag.

-> (b -> ConduitM Event o m c)

Handler function to handle the attributes and children of a tag, given the value return from the AttrParser

-> ConduitM Event o m (Maybe c)

The most generic way to parse a tag. It takes a predicate for checking if this is the correct tag name, an AttrParser for handling attributes, and then a parser for dealing with content.

This function automatically absorbs its balancing closing tag, and will throw an exception if not all of the attributes or child elements are consumed. If you want to allow extra attributes, see ignoreAttrs.

This function automatically ignores comments, instructions and whitespace.

tagPredicate

Arguments

:: MonadThrow m

=> (Name -> Bool)

Name predicate that returns True if the name matches the parser

-> AttrParser a

The attribute parser to be used for tags matching the predicate

-> (a -> ConduitM Event o m b)

Handler function to handle the attributes and children of a tag, given the value return from the AttrParser

-> ConduitM Event o m (Maybe b)

A simplified version of tag which matches against boolean predicates.

tagName

Arguments

:: MonadThrow m

=> Name

The tag name this parser matches to (includes namespaces)

-> AttrParser a

The attribute parser to be used for tags matching the predicate

-> (a -> ConduitM Event o m b)

Handler function to handle the attributes and children of a tag, given the value return from the AttrParser

-> ConduitM Event o m (Maybe b)

A simplified version of tag which matches for specific tag names instead of taking a predicate function. This is often sufficient, and when combined with OverloadedStrings and the IsString instance of Name, can prove to be very concise. . Note that Name is namespace sensitive. When using the IsString instance of name, use > "{http:/ab}c" :: Name to match the tag c in the XML namespace http://a/b

tagNoAttr

Arguments

:: MonadThrow m

=> Name

The name this parser matches to

-> ConduitM Event o m a

Handler function to handle the children of the matched tag

-> ConduitM Event o m (Maybe a)

A further simplified tag parser, which requires that no attributes exist.

content :: MonadThrow m => Consumer Event m Text

Grabs the next piece of content. If none if available, returns empty. This is simply a wrapper around contentMaybe.

contentMaybe :: MonadThrow m => Consumer Event m (Maybe Text)

Grabs the next piece of content if available. This function skips over any comments and instructions and concatenates all content until the next start or end tag.

Attribute parsing

data AttrParser a

A monad for parsing attributes. By default, it requires you to deal with all attributes present on an element, and will throw an exception if there are unhandled attributes. Use the requireAttr, optionalAttr et al functions for handling an attribute, and ignoreAttrs if you would like to skip the rest of the attributes on an element.

Alternative instance behave like First monoid. It chooses first parser which doesn't fail.

Instances

Alternative AttrParser

Monad AttrParser

Functor AttrParser

Applicative AttrParser

requireAttr :: Name -> AttrParser Text

Require that a certain attribute be present and return its value.

optionalAttr :: Name -> AttrParser (Maybe Text)

Return the value for an attribute if present.

requireAttrRaw :: String -> ((Name, [Content]) -> Maybe b) -> AttrParser b

optionalAttrRaw :: ((Name, [Content]) -> Maybe b) -> AttrParser (Maybe b)

ignoreAttrs :: AttrParser ()

Skip the remaining attributes on an element. Since this will clear the list of attributes, you must call this after any calls to requireAttr, optionalAttr, etc.

Combinators

orE

Arguments

:: Monad m

=> Consumer Event m (Maybe a)

The first (preferred) parser

-> Consumer Event m (Maybe a)

The second parser, only executed if the first parser fails

-> Consumer Event m (Maybe a)

Get the value of the first parser which returns Just. If no parsers succeed (i.e., return Just), this function returns Nothing.

orE a b = choose [a, b]

choose

Arguments

:: Monad m

=> [Consumer Event m (Maybe a)]

List of parsers that will be tried in order.

-> Consumer Event m (Maybe a)

Result of the first parser to succeed, or Nothing if no parser succeeded

Get the value of the first parser which returns Just. If no parsers succeed (i.e., return Just), this function returns Nothing.

many :: Monad m => Consumer Event m (Maybe a) -> Consumer Event m [a]

Keep parsing elements as long as the parser returns Just.

force

Arguments

:: MonadThrow m

=> String

Error message

-> ConduitM Event o m (Maybe a)

Optional parser to be forced

-> ConduitM Event o m a

Force an optional parser into a required parser. All of the tag functions, choose and many deal with Maybe parsers. Use this when you want to finally force something to happen.

Exceptions

data XmlException

Constructors

XmlException

Fields

xmlErrorMessage :: String

xmlBadInput :: Maybe Event

InvalidEndElement Name

InvalidEntity Text

UnparsedAttributes [(Name, [Content])]

Instances

Show XmlException

Exception XmlException

Typeable * XmlException

Other types

data PositionRange :: *

Instances

Eq PositionRange

Ord PositionRange

Show PositionRange

type EventPos = (Maybe PositionRange, Event)